WikiLeaks' donation portal through French non-profit FDNN was also taken offline, shortly after WikiLeaks linked to it as a way to donate while the WikiLeaks website is offline.

WikiLeaks offered their speculation of possible reasons for the timing of the attacks:

Attacking under the cover of the London Olympics.

An upcoming WikiLeaks release.

Ongoing releases of the Syria Files and the Global Intelligence Files.

A cable which reports on an assassination attempt on Chinese leader Hu Jintao's son and another killing at the top of the Chinese Communist Party. This cable was recently reported on by The Epoch Times and China's Forbidden News.

Death and Taxessuggested that the attack may be coming from the U.S. Government, or another government which holds an anti-WikiLeaks stance.

WikiLeaks and FDNN are back online. Although, they are not accessible by everyone.

UPDATE 2012-08-10 18:27 BST

Today, through a series of tweets, WikiLeaks explained in detail the attack they are facing:

More information on the ongoing DDoS attack against WikiLeaks related sites and donation infrastructure follows:

The attack is well over 10Gbits/second sustained on the main WikiLeaks domains.

The bandwidth is used is so huge it is impossible to filter without specialized hardware, however...

the DDoS is not simple bulk UDP or ICMP packet flooding, so most hardware filters won't work either.

The rage of IPs used is huge. Whoever is running it controls thousands of machines or is able to simulate them.

We have even tried moving behind http://Cloudflare.com but Cloudflare has re-emptively banned WikiLeaks. Living in the wild wild west.

Despite the ongoing DDoS attack, WikiLeaks continues to publish more Global Intelligence Files. The most reliable way to current access new releases is through the WikiLeaks .onion mirror at http://isax7s5yooqgelbr.onion/ (accessible only via Tor).

The DDoS attack against WikiLeaks has also gone after our donations infrastructure, the Fund for Network Neutrality, who also fund others. Attacks on WikiLeaks increase donations, so the more we need to fight, the more cash reserves we have to fight. But now that is prevented.